Fin-stabilised artillery shell

ABSTRACT

The present invention relates to a long-range artillery shell ( 1 ) that is fin-stabilised in its trajectory and which is designed to be fired in a rifled gun barrel and thus has a slipping driving band ( 8 ) as main contact surface with the inside of the barrel, and has a so-called base-bleed unit ( 3 ) with a number of stabilization fins ( 9 - 14 ) that are deployable after the shell has left the barrel. A special feature of the shell ( 1 ) as claimed in the present invention is that the fins ( 9 - 14 ), when activated, are radially displaceable to project outside the external periphery of the shell through slots or through-holes ( 28 ) in the wall of the shell, but are initially radially retracted inside the propellant motor section or propellant chamber ( 4 ) of the base-bleed unit ( 3 ) between dedicated protective walls ( 16 - 17 ) that isolate the fins from the surrounding propelling charges ( 25 ) of the propellant motor and also divide the inside of the propellant motor ( 4 ) into sectors ( 18 - 23 ) that are separated from each other.

[0001] The present invention relates to a long-range artillery shellthat is fin-stabilised in its trajectory and which is designed to befired in a rifled gun barrel and thus has a so-called slipping drivingband as main contact surface with the inside of the barrel and, after ithas left the barrel, has deployable stabilisation fins. Special featuresof the shell as claimed in the present invention are the design of thestabilisation fins, the way they are deployed, and the fact that whileretracted they are inside a propellant chamber or propellant motor of abase-bleed unit incorporated in the shell.

[0002] A possible reason for choosing a fin-stabilised artillery shellinstead of a spin-stabilised shell is that one could want to make itguideable on its way to the target and it is much easier to correct thetrajectory of a fin-stabilised shell than a spin-stabilised shell, andthis applies irrespective of whether the correction to the trajectory isto be achieved by impulse motors, guidance fins or some other method.

[0003] A requirement for the shell as claimed in the present inventionis that it shall be possible to provide it with extra long range. Amethod used increasingly in recent years to achieve extreme long rangeseven with old tube-firing artillery is the base-bleed technique used toeliminate rear-end turbulence and the underpressure formed behind shellsas they fly through the atmosphere, both of which have a deceleratingeffect that shortens range. The base-bleed technique involves theinstalling, in the rear section of the shell, of a combustion chamberfilled with slow burning propellant which, while it burns, generatesgases that flow out through an orifice in the rear face of the shell ata pre-determined rate, thus eliminating and equalising the deceleratingturbulence and underpressure behind the shell.

[0004] However, when providing a shell with a base-bleed unit as well asstabilisation fins there arises a problem regarding the location of thelatter as the base-bleed unit must be located in the rear of the shellwith at least one gas outflow outlet in the rear face of the shell,while the fins also need to be located in the rear section of the shellas far as possible from the centre of gravity of the shell. An extraproblem is that to enable the shell to be fired from a rifled barrel thefins must be fully retractable inside the minimum diameter of the barrelwhile not occupying too large a volume inside the shell therebypreventing the use of this space for the cargo that justifies theexistence of the shell.

[0005] The present invention now offers a solution to the problem withretractable fins that involves an advantageous function while they arein retracted mode and which enables location of the fins very close tothe rear face of the shell, i.e. at precisely the position where theyneed to be located.

[0006] As claimed in the present invention the fins are initiallyretracted radially or accommodated in the base-bleed unit's propellantchamber or motor section via slots or through-openings in its outerwall. In retracted mode the fins are thus enclosed by radial protectivewalls that remain in place even after the fins have deployed. Naturallythe protective walls and the spaces occupied by the fins occupy a smallpart of the total volume of the propellant chamber but, at the sametime, one obtains what can be considered a division of the propellantchamber into a number of sectors separated from each other by theprotective walls of the fin compartments while these sectors remain inmutual contact via a central axial space around the longitudinal axis ofthe propellant chamber that leads to the above mentioned gas outfloworifice. The present invention, namely, does not permit the fins and theprotective walls surrounding them in retracted mode to extend all theway to the central axis of the propellant chamber; instead, they areterminated just before this point.

[0007] This arrangement, as claimed in the present invention, providesseveral advantages. Firstly the fins are optimally located, i.e. at theextreme rear of the shell, and secondly the location of the fins inretracted mode does not impact negatively on the active cargo of theshell, and thirdly the location of the fins involves only a slightextension of the propellant chamber of the base-bleed unit to achievethe same volume that was previously available for an active propellantcargo, and finally by subdividing the propellant chamber into sectorsone obtains ‘free’ access to an efficient division and support of thebase-bleed propellant. The latter aspect has shown itself to be at leastas important, since previously there were major problems in producing abase-bleed unit suitable for slow-burning propellant elements ofsufficient size and strength to withstand the accelerations involved infiring while also holding them together until their active burnout.Consequently, it was previously necessary to devise special propellantsupports inside the combustion chamber of the base-bleed unit. Anexample of such a propellant support in the form of a support cupolainitially arranged internally around the outlet nozzle of a base-bleedunit is described in our own Swedish patent number 461477.

[0008] To provide the fins with a greater length than is immediatelyenabled by the diameter of the shell the fins can be given a telescopicfunction, i.e. each fin is produced in the form of two or moreinitially—before deployment—telescoped parts. To extend these finelements, both from their compartments between the protective wallsinside the propellant chamber and from each other, parts of the gaspressure that propels the shell from the barrel can be used in a waydescribed in more detail below. This gas pressure can subsequently, to agreater or lesser extent, also be supplemented by the gas pressuregenerated inside the propellant chamber of the base-bleed unit when thepropellant therein is ignited. The available gas pressure is thus usedto push the fins through their respective slots in the side wall of theshell and to extend them from their telescoped mode. To provide adesirable seal when the fins have reached their fully deployed positiontheir inner edges should preferably be designed so that they areslightly flared inwards towards the inside of the propellant chamber, sothat as soon as they have each reached their fully deployed positionthey become wedged firmly in their respective slots in the outer wall ofthe propellant chamber or become wedged/locked at the extremity of eachfirst fin element.

[0009] Of course, to provide extension of the telescoped fin elementsvarious completely mechanical devices, such as different types ofsprings, could alternatively be used. Even combinations of mechanicaland gas pressure controlled systems are fully conceivable within thefundamental concept of the present invention.

[0010] As indicated above parts of the gas pressure from the firing ofthe shell can be utilised to deploy the fins. Access to this propellantgas pressure is enabled by allowing it to enter the base-bleed unit,i.e. the unobstructed central passage of the propellant chamber. Whenthe shell exits the barrel from which it is fired there is thus also apressure inside the propellant chamber of the base-bleed unit that isequivalent to the pressure in the barrel. When the shell leaves thebarrel the pressure outside the shell rapidly drops to normalatmospheric pressure, while the pressure inside the propellant chamberdrops much more slowly as the sole opening of significance (to achievepressure equilibrium) is the gas outlet of the base-bleed unit. Thus itis between the time when the shell leaves the barrel and before thepressure inside the base-bleed unit has had time to reach equilibrium(with the ambient atmospheric pressure) that the overpressure availableis used to deploy the fins.

[0011] A special variant of the present invention utilises a removableprotective casing that protects and retains the fins in retracted modeuntil the shell has left the barrel after being fired. An elementary wayof mechanically removing this protective casing also involves using thegas pressure in the barrel during firing and allowing it free access tothe inside of the casing. When the shell reaches the muzzle a pressureequal to the pressure in the barrel also exists inside the protectivecover, but as soon as the shell exits the muzzle the pressure outsidethe cover rapidly drops to the ambient atmospheric pressure while thepressure inside the protective cover falls more slowly, resulting inthis internal overpressure ejecting the protective cover against thesole smaller resistance offered by the atmospheric pressure. As alreadydescribed, the same internal overpressure can also be used to deploy thefins.

[0012] Radially retracted fins have, of course, existed previously, butas far as we are aware they have never been directly retracted into thepropellant chamber of a base-bleed unit in the way described in thepresent invention, where the fins in retracted mode are also protectedby radial support guide-walls that have the double function of acting asactive propellant supports.

[0013] The present invention is defined in the subsequent patent claimsand is now described in more detail with reference to the illustrationsshown in the appended FIGS. 1-5.

[0014] In the appended figures

[0015]FIG. 1 shows a sectioned shell equipped with the characteristicfins, while

[0016]FIG. 2 shows to a larger scale a longitudinal section of theshell's base-bleed unit in pre-launch mode, and

[0017]FIG. 3 shows a section along plane III-III in FIG. 2, while

[0018]FIGS. 4 & 5 show the same projection as FIG. 3 during differentphases of fin deployment.

[0019] The shell 1 illustrated in FIG. 1 has a front section 2 that cancontain a fuze, arming and safety functions, control functions andcargo. These parts are not part of the present invention and will thusnot be commented on further. In the rear section of the shell 1 there isa base-bleed unit with the general designation 3. Immediately in frontof the base-bleed unit 3 there is a groove in the shell body in whichthe slipping plastic driving band 8 is mounted. The base-bleed unit 3contains a propellant chamber 4 and a centrally located gas outlet 5.The shell 1 is also equipped with a number of deployable fins 9-14 thatare shown in deployed mode in FIGS. 1 and 5, and in retracted mode inFIGS. 2, 3 and 4. Each of the fins consists of an inner primary fin 6retracted in the shell body or, more precisely, in the base-bleed unit3, and a telescopic secondary fin 7 retracted/telescoped into the saidprimary fin. Each of the primary fins 6 is radially guided and radiallydisplaceable between supporting, protective walls 16 and 17 respectively(see FIG. 3) arranged on each side of each said primary fin, and as theinner longitudinal edges 15 of the primary fins 6 also have free contactwith the inside of the propellant chamber 4, as soon as the primary finsleave the barrel they are pressed outwards to deploy through theirrespective slots 28 in the wall of the shell body in the way previouslydescribed by the remaining pressure from the barrel phase, possiblysupplemented by the pressure from the newly ignited base-bleedpropellant. In a corresponding way the secondary fins 7 are mounteddisplaceably in the primary fins 6, and are also dependent on propellantgas pressure in the propellant chamber 4 for deployment. Until shell 1has left the barrel of the gun from which it is fired by a certainmargin, the base-bleed unit and the retracted fins are covered by aprotective casing 26. As illustrated in FIG. 2 the protective casing 26initially covers the rear section of the shell and thereby retains thefins in retracted mode. This mode is shown in FIG. 2. By giving the gaspressure that propels the shell during the actual firing free access tothe inside of the protective casing 26 via a separate opening 27 in thesaid casing, a high overpressure is generated inside the protectivecasing 26 but when the said shell exits the muzzle of the gun fired thepressure outside the protective casing 26 falls extremely rapidly whilethe pressure inside the protective casing cannot possibly fall equallyrapidly. The result is that the overpressure inside the protectivecasing 26 becomes so great that it ejects the said casing rearwards fromthe outside of the base-bleed unit 3 as illustrated in FIGS. 4 and 5.

[0020] Simultaneously with, or immediately after, ejection of theprotective casing 26 the propellant charge of the base-bleed unit 3 isinitiated and the remaining pressure from the barrel phase issimultaneously used to force the primary and secondary fins 6 and 7outwards to deploy. When the primary fins 6 reach their respectiveoutermost position their respective inner longitudinal edges 15 seal theslots in the wall of the base-bleed unit through which the said primaryfins deployed, while the gas pressure also deploys the secondary fins 7to a correspondingly sealed and locked outer position.

[0021] As illustrated primarily in FIG. 3 the primary fins 6 inretracted mode are enclosed on both sides by the previously mentionedsupporting, protective walls 16 and 17 that form an integral temperatureresistant lining of the propellant chamber 4 of the base-bleed unit,such that the pair of supporting, protective walls of each two adjacentfins divide the propellant chamber 4 into a number of sectors orsegments designated 18-23 in the figures, each such sector initiallycontaining a dedicated quantity of propellant or propellant body 25.Extending through the base-bleed unit 3 there is a central propellantgas and initiation channel 24 that is common to all the sectors 18-23 ofthe propellant chamber 4 as each such sector is exposed to the saidchannel.

[0022] As each of the propellant sectors 18-23 are restricted in size inthis way and are provided with good lateral support by the protectivewalls 16-17 of the adjacent fins 9-14 it is possible to eliminate anyrisk of damage to the propellant charge of the base-bleed unit duringfiring, i.e. before it comes into use, while this subdivision intosectors also enables good strength properties for the propellant bodiesright up to burnout.

We hereby claim and desire to secure by Letters Patent the following: 1.An artillery shell (1) of the type that incorporates a base-bleed unit(3) to increase the range of the shell and is equipped with fins (9-14)for stabilisation in trajectory, wherein the fins (9-14) when activated,are radially displaceable to project outside the external periphery ofthe shell through slots or through-holes (28) in the wall of the shell,but are initially radially retracted in the propellant motor section orpropellant chamber (4) of the base-bleed unit (3) between dedicatedprotective walls (16-17) which isolate the fins from the surroundingpropelling charges (25) of the propellant motor and also divide theinside of the propellant motor into sectors (18-23) that are separatedfrom each other.
 2. An artillery shell (1) as claimed in claim 1 whereinthe fins (9-14) that are initially radially retracted and theirsurrounding protective walls (16, 17) leave a central propellant gaschannel free in the centre of the propellant chamber (4) even when thesaid fins are in retracted mode.
 3. An artillery shell (1) as claimed ineither of claims 1 or 2 wherein each fin (9-14) is divided into two ormore telescopically retractable elements (6-7).
 4. An artillery shell(1) as claimed in any of claims 1-3 wherein the shell, even when thefins consist of several secondary (7) and primary elements (6)telescopically retractable in each other, deploys the fins after theshell has left the barrel by utilising the residual barrel pressureinside the propellant chamber (4) of the base-bleed unit (3)alternatively supplemented by the propellant gas pressure from thepropellant charge contained in the said chamber when the charge isignited.
 5. An artillery shell (1) as claimed in any of claims 1-4wherein until it has left the barrel from which it is fired the saidshell has a protective casing (26) covering the retracted fins (9-14)and the base-bleed unit (3) that is removable rearwards, in relation tothe direction of flight, which casing via a dedicated opening (27),preferably concentric with the gas outlet (5) from the base-bleed unit(3), has access to the overpressure prevailing during the firing orbarrel phase in the barrel of the weapon in which the shell is fired. 6.An artillery shell (1) as claimed in any of claims 1-5 wherein the innerlongitudinal edges (15) of the fin elements (6, 7) facing the propellantchamber (4), when deployed in their outermost position, close theirrespective slots (28) in the propellant chamber (4) of the base-bleedunit (3) and close their respective slots in the outer edge of theprimary fins (6) through which they deploy.